The present invention relates in general to rheometers for measuring the flow of viscous fluids, and in particular to a new and useful elongation rheometer which is useful in measuring the elongational viscosity of a polymeric solution.
Commercially available tensile testing equipment can be used in measuring the elongational viscosity of viscous polymer melts. In such equipment a rod-like sample of the polymer melt is suspended in a silicone oil bath to compensate for gravity by using a piece of the sample, and then the sample is stretched at a constant tensile stress or constant strain rate. This equipment measures the force necessary to stretch the rod-like sample at the constant strain rate in order to determine the elongational viscosity thereof.
Polymer solutions however, have viscosities which are far too low to form a stable rod-like sample and therefore cannot be tested using commercially available tensile testing machines. It is important however, to measure the elongational viscosity of polymer solutions because they play a significant role in many processes such as atomization. For example, when Newtonian liquids are subjected to relatively high air velocities, small particles normally result. To increase the drop size of the atomized fluid, polymers are often added. With this addition of polymer, non-Newtonian viscoelastic solutions are produced.
Various techniques have been reported which investigates the stretching flow of low viscosity solutions. See for example Petrie, C. J. S. (1979), Elongational Flows, Academic Press, New York and London. This article gives an excellent summary of these experiments and their particular limitations. Also see Jones, W. M. and Rees, I. J. (1982), The Stringiness of Dilute Polymer Solutions, J. Non-Newtonian Fluid Mechanics, II, 257-268. This article describes a falling drop experiment to estimate the apparent elongational viscosity of dilute polymer solutions. An apparent elongational viscosity is reported since the elongation rate varied throughout the stretching process.